Zombie Processes & Prevention


  • 5
    A

    When a process is created in UNIX using fork() system call, the address space of the Parent process is replicated. If the parent process calls wait() system call, then the execution of parent is suspended until the child is terminated. At the termination of the child, a ‘SIGCHLD’ signal is generated which is delivered to the parent by the kernel. Parent, on receipt of ‘SIGCHLD’ reaps the status of the child from the process table. Even though, the child is terminated, there is an entry in the process table corresponding to the child where the status is stored. When parent collects the status, this entry is deleted. Thus, all the traces of the child process are removed from the system. If the parent decides not to wait for the child’s termination and it executes its subsequent task, then at the termination of the child, the exit status is not read. Hence, there remains an entry in the process table even after the termination of the child. This state of the child process is known as the Zombie state.

    Different ways in which creation of Zombie can be prevented:-

    1. Using wait() system call : When the parent process calls wait(), after the creation of child, it indicates that, it will wait for the child to complete and it will reap the exit status of the child. The parent process is suspended(waits in a waiting queue) until the child is terminated. It must be understood that during this period, the parent process does nothing just waits.
    // A C program to demonstrate working of
    // fork()/wait() and Zombie processes
    #include<stdio.h>
    #include<unistd.h>
    #include<sys/wait.h>
    #include<sys/types.h>
     
    int main()
    {
        int i;
        int pid = fork();
        if (pid==0)
        {
            for (i=0; i<20; i++)
                printf("I am Child\n");
        }
        else
        {
            wait(NULL);
            printf("I am Parent\n");
            while(1);
        }
    }
    
    1. By ignoring the SIGCHLD signal : When a child is terminated, a corresponding SIGCHLD signal is delivered to the parent, if we call the ‘signal(SIGCHLD,SIG_IGN)’, then the SIGCHLD signal is ignored by the system, and the child process entry is deleted from the process table. Thus, no zombie is created. However, in this case, the parent cannot know about the exit status of the child.
    // A C program to demonstrate ignoring 
    // SIGCHLD signal to prevent Zombie processes
    #include<stdio.h>
    #include<unistd.h>
    #include<sys/wait.h>
    #include<sys/types.h>
     
    int main()
    {
        int i;
        int pid = fork();
        if (pid == 0)
            for (i=0; i<20; i++)
                printf("I am Child\n");
        else
        {
            signal(SIGCHLD,SIG_IGN);
            printf("I am Parent\n");
            while(1);
        }
    }
    
    1. By using a signal handler : The parent process installs a signal handler for the SIGCHLD signal. The signal handler calls wait() system call within it. In this senario, when the child terminated, the SIGCHLD is delivered to the parent.On receipt of SIGCHLD, the corresponding handler is activated, which in turn calls the wait() system call. Hence, the parent collects the exit status almost immediately and the child entry in the process table is cleared. Thus no zombie is created.
    // A C program to demonstrate handling of
    // SIGCHLD signal to prevent Zombie processes.
    #include<stdio.h>
    #include<unistd.h>
    #include<sys/wait.h>
    #include<sys/types.h>
     
    void func(int signum)
    {
        wait(NULL);
    }
     
    int main()
    {
        int i;
        int pid = fork();
        if (pid == 0)
            for (i=0; i<20; i++)
                printf("I am Child\n");
        else
        {
            signal(SIGCHLD, func);
            printf("I am Parent\n");
            while(1);
        }
    }
    

  • 0
    A

    hope to add it soon


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